(C#) Understanding a few ECDSA Public Key Formats

See more ECC Examples

Describes a few ECDSA public key formats.

Chilkat .NET Downloads Chilkat .NET Framework Chilkat for .NET Core

// Here we have the output of the following openssl command:  openssl ec -in key.pem -pubout -text

// Private-Key: (256 bit)
// priv:
//     0e:63:25:8a:73:3c:71:b6:c0:e7:a3:0f:94:b9:74:
//     e0:be:bd:46:18:be:40:7e:66:9e:21:99:85:0e:ed:
//     87:2d
// pub:
//     04:5d:1a:4f:d9:bd:49:9e:e4:fd:55:2c:0d:ea:6d:
//     b1:66:64:7a:71:91:13:63:86:fe:ca:94:d4:47:51:
//     39:66:ff:43:d5:62:de:f2:f2:41:3c:2e:3f:95:18:
//     2d:23:f7:e7:8e:75:19:3b:c6:50:fb:d9:90:f5:e8:
//     12:b7:b8:6a:43
// ASN1 OID: prime256v1
// NIST CURVE: P-256
// writing EC key
// -----BEGIN PUBLIC KEY-----
// MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET
// Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==
// -----END PUBLIC KEY-----

// The public key is shown in two different formats.
// The first is this:

// pub:
//     04:5d:1a:4f:d9:bd:49:9e:e4:fd:55:2c:0d:ea:6d:
//     b1:66:64:7a:71:91:13:63:86:fe:ca:94:d4:47:51:
//     39:66:ff:43:d5:62:de:f2:f2:41:3c:2e:3f:95:18:
//     2d:23:f7:e7:8e:75:19:3b:c6:50:fb:d9:90:f5:e8:
//     12:b7:b8:6a:43

// It is the ANSI X9.63 format.
// 65-bytes are the uncompressed public key (04 || X || Y) 

// This is the same public key, but in PEM format

// -----BEGIN PUBLIC KEY-----
// MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET
// Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==
// -----END PUBLIC KEY-----

// It contains ASN.1 that more explicitly identifies the key type.
// 
// SEQUENCE (2 elem)
//   SEQUENCE (2 elem)
//     OBJECT IDENTIFIER 1.2.840.10045.2.1 ecPublicKey (ANSI X9.62 public key type)
//     OBJECT IDENTIFIER 1.2.840.10045.3.1.7 prime256v1 (ANSI X9.62 named elliptic curve)
//   BIT STRING (520 bit) 000001000101110100011010010011111101100110111101010010011001111011100...

bool success;

// PEM format can be loaded into a Chilkat public key object like this:

Chilkat.StringBuilder sbPem = new Chilkat.StringBuilder();
sbPem.Append("-----BEGIN PUBLIC KEY-----\r\n");
sbPem.Append("MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET\r\n");
sbPem.Append("Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==\r\n");
sbPem.Append("-----END PUBLIC KEY-----\r\n");

Chilkat.PublicKey pubKey1 = new Chilkat.PublicKey();
success = pubKey1.LoadFromString(sbPem.GetAsString());
if (success == false) {
    Debug.WriteLine(pubKey1.LastErrorText);
    return;
}

// The X9.63 format can be loaded like this:
Chilkat.StringBuilder sbHex = new Chilkat.StringBuilder();
sbHex.Append("04:5d:1a:4f:d9:bd:49:9e:e4:fd:55:2c:0d:ea:6d:");
sbHex.Append("b1:66:64:7a:71:91:13:63:86:fe:ca:94:d4:47:51:");
sbHex.Append("39:66:ff:43:d5:62:de:f2:f2:41:3c:2e:3f:95:18:");
sbHex.Append("2d:23:f7:e7:8e:75:19:3b:c6:50:fb:d9:90:f5:e8:");
sbHex.Append("12:b7:b8:6a:43");

// Get rid of the ":" chars.
int numReplaced = sbHex.Replace(":","");

// We'll need to convert hex to base64..
Chilkat.BinData bdKey = new Chilkat.BinData();
bdKey.AppendEncoded(sbHex.GetAsString(),"hex");

Chilkat.PublicKey pubKey2 = new Chilkat.PublicKey();
success = pubKey2.LoadFromString(bdKey.GetEncoded("base64"));
if (success == false) {
    Debug.WriteLine(pubKey2.LastErrorText);
    return;
}

// Let's get the key in pubKey2 as PEM.
// It should be idential to the PEM above.
Debug.WriteLine(pubKey2.GetPem(true));

// Here's the output.  You can see it's the same as the PEM above..

// -----BEGIN PUBLIC KEY-----
// MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEXRpP2b1JnuT9VSwN6m2xZmR6cZET
// Y4b+ypTUR1E5Zv9D1WLe8vJBPC4/lRgtI/fnjnUZO8ZQ+9mQ9egSt7hqQw==
// -----END PUBLIC KEY-----